As semiconductor technology has advanced, the amount and speed of logic available on an integrated circuit (IC) has increased. As a result, ICs are consuming more power. The more power that is consumed, however, the greater the heat that is generated. Conventionally, ICs include devices such as heat sinks to absorb and dissipate heat. A heat sink is an article that absorbs and dissipates heat from an IC using thermal contact. For conventional ICs, heat sinks are thermally coupled to the face side of the die. For flip-chip mounted ICs, heat sinks are thermally coupled to the backside of the die. Heat sinks are typically attached to ICs using a thermal paste. The term “face side” denotes the side of an IC die that receives the bulk of semiconductor processing such that circuitry and interconnect are fabricated on that face side. The backside is opposite the face side of the die.
For a flip-chip IC, for example, the primary heat removal path is through the backside of the die, where a heat sink is attached. Heat is dissipated through several mechanisms, including: (1) vertical heat conduction to the backside of the die and to the heat sink; (2) vertical heat conduction through the die, as well as lateral heat conduction within the base of the heat sink and thermal paste (i.e., heat spreading); and (3) heat convection to the ambient environment. Lateral heat conduction in item (2) depends strongly on the ratio between die area and heat sink base area. When estimating the thermal resistance of a flip-chip package with a heat sink, engineers must account for the spreading resistance (a thermal resistance). The higher the ratio between heat sink base area and die area, the higher the spreading resistance.
The increase in the speed and amount of logic on an IC has outpaced the number and performance of input/output (I/O) connections. As a result, IC die stacking techniques have received renewed interest to address the interconnection bottleneck of high-performance systems. In stacked IC applications, two or more ICs are stacked vertically and interconnections are made between them. One approach to IC stacking involves mounting a second die on the backside of a first die. The stacked IC arrangement is then flip-chip mounted/packaged. A heat sink is then attached to the stacked die or dice.
When a die or dice are stacked on the backside of an IC, the thermal design of the IC may be compromised. For example, if stacked IC dice occupy a total area smaller than the area of the primary IC, there are additional components to spreading resistance. One such component is due to the interface between the primary IC die and the stacked die or dice. Another such component is due to the interface between the stacked die or dice and the heat sink. These additional spreading resistance components lead to poor thermal design and higher junction-to-package thermal resistance. Accordingly, there exists a need in the art for a semiconductor assembly having reduced thermal spreading resistance and methods of making the same.